The word "volumosity" is not a commonly used word in English, but it exists. Its spelling can be explained through its International Phonetic Alphabet (IPA) transcription as "vəʊˌljuːˈmɒsɪti". The "vow" sound at the beginning represents the "vo" syllable while the "lu" sound indicates the "lum" syllable. The "osi" syllable is pronounced with a long "o" in the first sound and a short "i" sound in the second. The stress is on the second syllable (-lume-) in this word. Overall, "volumosity" is a complex word that is not much in use but can be heard occasionally.
The vast bulk of printed matter in volumes.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.